1. Field of the Invention
The invention relates to the field of methods for the manufacture of electromagnetic waveguides. It can be used for any waveguide, but is particularly suited to non-rectilinear waveguides, especially certain antennas designed in the form of waveguide radiators.
2. Description of the Prior Art
It is known that, owing to dimensional tolerances, the idea of manufacturing such waveguides directly by casting has had to be given up. The casting tolerances, which are at best in the range corresponding to the JS13 or JS14 standards, cannot give the dimensional precision required for a waveguide. These castings therefore have to be machined to bring them to the exact dimensions desired. This entails substantial costs. Similarly, attempts have been made to obtain waveguides directly by casting through operations for the selection of the stripped-out waveguides. Reject rates of the order of 70% make the operation uneconomical.
The aim of the invention is to make waveguides out of castings. It is aimed at simplifying the intermediate operations to be carried out between the rough-cast waveguide and the finished waveguide.
To this end, according to the present invention, it is provided that the casting will be given a shape that will make it possible, by simple chasing or peening of material, burnishing or rolling, possibly in certain cases abrasion by a conformationally shaped tool or broaching to obtain the finished waveguide directly.
It is known that a waveguide is a sort of pipe whose cross-section is, for example, a rectangle or a circle. The invention here below will be described for a rectangular-sectioned waveguide but the explanations given will enable the invention to be easily transposed to any other shape of waveguide. The rectangular sectioned waveguides have a large side with a length a and a small side with a length b.
It is generally sought to obtain a single mode of propagation of the wave, known as the transverse mode TE 10. To this end, the lengths a and b must verify the relationship: ##EQU1## an expression in which .lambda. designates the wavelength of the waveguide. The value generally taken is a=.lambda..sqroot.2. Furthermore, in order to limit the losses in the waveguide, the state of the surface should be such that the differences between the bumps and the hollows do not exceed .lambda./4, which is the boundary value. In practice it is sought to limit these differences to .lambda./8 or even .lambda./10. Thus, for a waveguide working in the 20 GHz band, i.e. with 15 mm wavelengths, the differences between hollows and bumps will be limited to 20 tenths or at best to 15 tenths of a millimeter.
To meet this condition, according to the invention a molding operation will be used to create a surface state comprising bumps and hollows, and then the material of the bumps will be chased into the hollows. Since these are longitudinal elements, the bumps and the hollows will, in most cases, have shapes that are also longitudinal. They will be grooves whose cross-sections could have trapezoidal, triangular, rectangular or undulating shapes. The dimensions of the hollows and bumps shall now be explained. Let X be the size to be obtained with a tolerance value of x. This will be the size, for example, of the large side of a rectangular waveguide. In this example, it will be assumed that the final size is obtained by the chasing of material. The size X is a size corresponding to a female element, i.e. if the real size Xr is smaller than X, then it is necessary to remove or chase material in order to obtain the size X. In a known way, the chasing of material should be done on a minimum thickness e of material. The caster's tolerance value is f. With these designations, the minimum distance between facing bumps to be given to the caster will be equal to: EQU CF Min=X+x-e-f
The maximum distance between bumps will therefore be: EQU CF Max=X+x-e
The method is applicable only if f&gt;x. If not, it would mean that a rough casting is enough.